When I press the A/C button the red light turns on but only as long as I’m pushing it in. As soon as I let go it turns off and A/C never comes on. Any suggestions?

 

Low hanging fruit approach is to press the 'AUTO' button on the HVAC and see what happens; it should turn on the A/C when in AUTO mode, if the A/C system does not have a fault in it currently.

You didn't mention any prior A/C or HVAC issues, so that's where to start...simple. You can also just check other mechanical basics like the condition of your serpentine belt (it's still on there, for example). The A/C also isn't gonna engage if the pressure is too high or too low inside the system, and a bad pressure sensor/ switch can also keep it from functioning by providing false values as feedback, etc.

If that doesn't work or you have been experiencing HVAC or A/C problems, then either go pay a professional mechanic shop, or if you DIY, then hook up a VAGCOM scan tool and do a full scan of the car. It's a lot cheaper to buy the VAGCOM scan tool than pay a shop to scan your car / diagnose it even once, and while on topic, if you want to be able to DIY repairs get a good VAGCOM scan tool that will save you both money and...time, and help you diagnose the car properly. I'd recommend the Ross Tech VCDS hands down for advanced troubleshooting and what you can do with it. It's about $200 USD, but that's a one time cost for full, three VIN# access, and you get their support, access, WIKI, forum discussions free, etc.

You can get your car scanned at most auto parts stores, but if they don't have a VAGCOM capable scan tool (some do...ask), then it would only show problems that illuminate the CEL on dash, as relates to the type of scan tool they hook up can't read all the control modules in a VW Auto Group (VAGCOM) vehicle.

 

I tried setting it to “Auto” and the AC does sound as if it kicks in for a second but then it immediately stops. And no, I’ve never had any HVAC issues in the past on this vehicle.

 

That's good news if you heard the compressor clutch engage briefly; no A/C problems in the past and now it's just started this probably means it is just a sensor failure. The high-pressure A/C sensor is a known failure point on the Q7. Before I'd go throwing parts at it though, I'd scan the car for faults/trouble codes internally, which again, requires a VAGCOM type scan tool be used to see everything your car has to tell you. Q7s that live/lived in Wintry environments often will get pinhole leaks in A/C lines up around the radiator area where they get exposed to salt/road chemicals, etc.

If you don't DIY, I'd still suggest you get a cheap one just so you can read the fault codes on your Q7, as that is important on these older Q7s, since a LOT of issues don't cause the CEL or MIL to illuminate. You could get a basic Carista or OBD-eleven. The basic version of those is around $50 or less, but the device can sell for double or more with 'paywall' access included to provide additional access/functions, etc. Beware the paywall upgrades, especially if you don't DIY...not good tools for diagnostics/repair support, but great for just reading & clearing fault codes. I have one I keep in car just for that purpose now...relegated to code reading duty after I found out it sucks at actual diagnostics, LOL.

 

Thanks so much, I really appreciate the time you’ve taken to help me out. It’s very generous of you and I will heed your advice!

 

Can anyone show me the location of the AC high pressure switch? I have a 2010 Q7 TDI and I just can’t find the doggone thing!

 

The sensor is located beneath the windshield inside the plenum area. For LHD cars it is on the side where air intake is located...so the passenger side, in the plenum. You gotta remove the plastic plenum cover to get at it.

While it could be your issue, there are several other easy things to verify first, like are both radiator fans running when the A/C switch is ON, etc. Is there freon in system and at correct pressure for it to work, etc. The system turning off can be result of a bad sensor, but also from lack of freon, etc.

If that sensor itself is bad, there's normally gonna be a fault code showing for it. Scan the car using VAGCOM scan tool to see all the internal fault codes, which don't trigger the CEL/MIL lamp on dash or a driver's message, etc..

Here's a related thread:

https://www.audiworld.com/forums/q7-...witch-2950073/

 

It turns out my A/C has no pressure and that’s why it’s not turning on. From what I’ve seen, the A/C needs to be running in order to recharge it, but I can’t do that. Is it safe to just pump in a bit of refrigerant first in order to get it running and then top it off?

 

Quantify "my A/C has no pressure", as if it's empty or super low, then there's a leaking component to identify. Also, when it's super low, the system needs to be vacuumed out to remove any moisture and all the old compressor oil and freon remaining in system, then the vacuum is turned off and you see if it 'leaks down' or holds the vacuum in system...you want it to be able to hold the vacuum as that means there are no leaks to deal with. Then the system can be recharged with correct amount of freon and correct amount of new pag oil (compressor oil) that the system calls for.

If it's got freon in it, but is just a bit lower pressure than is required for the compressor to run, then IMO, that would be the only scenario you'd want to just add some refrigerant using a set of A/C manifold gauges to do it correctly, or you can buy a recharger kit w/gauge attached at local parts store, etc., and just wing it/hope for the best. Overcharging the system is much worse than it being a little bit low from aging seals, so be sure you know what you doing.

I bought a set of A/C manifold gauges at Harbor Freight for our Q7s, but you can also get them as a loaner tool from most major Auto parts stores too.